1. Field of the Invention
The present invention relates to techniques for forming oxides on silicon for use as semiconductor devices. More particularly, the present invention relates to methods for forming and annealing oxides in intermittent steps.
2. Description of the Related Art
In conventional semiconductor device processing schemes, oxidizing environments create oxide layers on silicon or other substrates. The oxidizing environments are typically created in wafer batch processors such as horizontal or vertical furnaces or in single wafer rapid thermal processors. The oxide is conventionally grown in a single step of a multi-step process. In the horizontal or vertical furnaces, the wafers are loaded into quartz, polysilicon, or silicon carbide boats and exposed to the oxidizing ambient in the furnace for a predetermined period of time. The oxidizing environment is purged by displacement using an inert gas such as nitrogen or argon, when the desired oxide thickness is achieved. The oxidizing environment conventionally performs one of dry oxidation (O2), wet oxidation (O2+H2), dry oxidation with a chlorine source (O2+HCl), and nitric oxide oxidation (NO), depending on the characteristics sought for the oxide.
An anneal step is typically performed at the end of the oxidation, after the final oxide thickness is achieved. During the oxidation step, stresses are built up in the oxide film, particularly at the oxide-silicon interface. Annealing provides stress relief for the oxide. However, particularly as the critical dimensions of semiconductor devices decrease, conventional methods may not satisfactorily reduce the stress levels in the oxide and at the oxide-silicon interface. Defects in thinner oxides tend to occur at the silicon-(substrate)-oxide interface. This contributes to higher defect levels in the oxides than desired, i.e. an unacceptably low oxide integrity and may produce an unreliable gate oxide. One measure of the reliability and the quality of the oxide is the Time Dependent Dielectric Breakdown (TDDB) lifetime which is related to the time required before a voltage applied to a gate oxide causes the breakdown of the oxide. Higher integrity oxides exhibit longer TDDB lifetimes and are often coveted by customers
Accordingly, it is desirable to provide an improved oxidizing process that will provide higher oxide integrity and more reliable devices.
To achieve the foregoing, the present invention provides a process of forming an oxide on a substrate in increments. In the process, wafer(s) are loaded into a thermal reactor and exposed to an oxidizing ambient for a predetermined period of time. The predetermined period is selected such that the oxide layer formed falls short of the desired oxide thickness. The oxidizing ambient is then turned off and an inert gas purge is introduced into the reactor. The wafers are then subjected to an annealing step in the reactor at a selected temperature, thus allowing stress relief The process of oxidizing and annealing in increments (xe2x80x9cpulsed oxidationxe2x80x9d) is repeated until the desired oxide thickness is achieved. The process permits the oxide layer to be grown and annealed in increments, thus permitting stress relief to occur at one or more stages before the final oxide thickness is achieved.
In one aspect, the invention provides a method of forming an oxide on a semiconductor substrate. A dielectric is grown at a substrate interface in a plurality of increments. Stress is relieved at the dielectric substrate interface between each increment.
In another aspect, stress relief is performed by annealing the substrate. The annealing is performed by placing the substrate in an inert environment and by raising the temperature surrounding the substrate.
In yet another aspect, growing the dielectric comprises exposing the substrate to an oxidizing ambient. The oxidizing ambient comprises one of dry oxidation (O2), wet oxidation (O2+H2), dry oxidation with a chlorine source, nitrous oxide (N2O) and nitric oxide (NO). The use of N2O or NO is dependent on the desired characteristics of the device performance.
In yet another aspect, a first increment of growing the dielectric comprises exposing the substrate to a first oxidizing ambient and a second increment comprises exposing the substrate to a second oxidizing ambient. The first oxidizing ambient is different from the second oxidizing ambient.
In yet another aspect, the first oxidizing ambient is the same as the second oxidizing ambient.